1. Field of the Invention
The present invention relates to a radio communication system, a base station, a relay station, a mobile station, and a packet transmission control method to which the multihop technology of establishing connection between a mobile station and a base station through multi-step relaying by relay stations, is applied.
In the radio communication system according to the present invention, stations associated with one communication operation include three stations, “base station,” “relay station,” and “mobile station,” wherein the “base station” will refer to a station that can be linked with another base station through core network, the “mobile station” to a terminal that first requests transmission of a packet or that receives a packet last (i.e., a final destination), and the “relay station” to a terminal that functions to relay a packet from the mobile station to the base station or to relay a packet from the base station to the mobile station.
The term “transmitting direction of a packet” generally refers to an “uplink direction” from the mobile station to the base station and a “downlink direction” from the base station to the mobile station in the above radio communication system, and the “uplink direction” and “downlink direction” will also be used under the above meanings. The term “hop count” means the number of relays carried out by stations existing between a transmission-originating station and the destination station. For example, in the case of communication in the uplink direction, the hop count means the number of relays carried out by mobile stations or relay stations between the base station and the mobile station when the mobile station performs the communication; specifically, the hop count of the base station is 0, the hop count of a mobile station capable of direct communication therewith is 1, and the hop count of a mobile station or a relay station other than those is assumed to increase by one per relay. The hop count in the embodiments of the invention described hereinafter means the foregoing “number of relays carried out by mobile stations or relay stations between the base station and the mobile station when the mobile station performs the communication.” In the case of communication in the downlink direction, the hop count is the number of relays carried out by base stations or relay stations between the base station and the mobile station when the base station performs the communication.
2. Related Background Art
In the conventional cellular mobile communication systems, a mobile station selects a base station yielding the highest reception level at every moment and establishes direct communication with the selected base station to participate in a network. On that occasion, if the reception level from the base station transmitting signals at the maximum power is below a required value, the mobile station will fail to establish communication with the base station.
In order to solve this problem, there are proposals for the multihop system of relaying signals via other mobile stations to establish communication with the base station on the occasion of failure in direct communication with the base station. For example, the mobile station 14 shown in FIG. 1 is able to establish direct communication with the base station 12, but the mobile station 11 is unable to establish direct communication with the base station 12, because an obstruction 15 exists between the mobile station 11 and the base station 12. The multihop technology is a technique wherein, in the case of the failure in direct communication, another mobile station 13 having a relaying function acts as a relay station to relay communication between the mobile station 11 and the base station 12, thereby implementing the communication between the mobile station 11 and the base station 12.
Since the multihop system as described enables communication of the mobile station lying in an area in which radio waves from the base station are unreachable, it facilitates installation of stations used to be hard to install because of buildings and topographic features. In the conventional methods the entire service area had to be covered by only cells of base stations, whereas adoption of the multihop system enables the mobile stations to establish communication by multihop even if there are gaps between cells. Therefore, the multihop system makes it feasible to decrease the number of base stations and to reduce cost necessary for construction of networks.
However, the CSMA base multihop system proposed heretofore suffered the so-called hidden terminal problem (the problem that, because adjacent stations transmit signals at the same frequency, a station lying in an overlapping area of cells of these adjacent stations fails to receive signals because of collision of received radio waves from the stations (e.g., reference is made to the report “Title: “Does IEEE 802.11 MAC protocol work well on multihop wireless ad-hoc networks,” Author: Shugong Xu, et al.” published in “IEEE communication magazine (2001)”)), and problems of increase of traffic and decrease of throughput of the system due to the hidden terminal problem and others still remained unsolved.
In the conventional cellular mobile communication systems, as described above, the range of the service area was largely affected by arrangement of base stations and there arose the problem that the mobile stations failed to establish communication in areas where reception levels of radio waves from the base stations were low. The conventional CSMA and TDMA base multihop systems are promising as means for enabling the above incommunicable mobile stations to establish communication, but the hidden terminal problem and others occurred depending upon arrangement of relay stations, so as to cause increase of traffic and decrease of throughput of the system.
The present invention has been accomplished in order to solve the above problems, and an object of the invention is to provide a radio communication system, a base station, a relay station, a mobile station, and a packet transmission control method that enable a mobile station to establish communication more securely independently of arrangement of relay stations while avoiding the increase of traffic and the decrease of throughput of the system.